The position or setting of tapes in magnetic tape recorders can be determined by the counting of pulses which are derived from a part moving together with the magnetic tape such as, for example, the spindles. The number counted may be employed for indicating the tape setting. Since the length of the circumference of a tape reel varies with the coiling or uncoiling of the magnetic tapes this indication is not linear. That is, considerably varying values are obtained for the tape section assigned to individual counting steps as a function of the tape setting.
With a view towards improving the linearity of indication it has been proposed (German Auslegeschrift No. 24 16 060.9) to provide a scanning unit on each of the two spindles, to derive tape travelling pulses from the scanning units and to feed the sum of the pulses into a counter. While this method effects some improvement in indication it is in some cases still inadequate in effecting an improvement in linearity such as, for example, when the indication has to be calibrated to the length of tape which has been run dimensionally.